THE A DA P TA TIONS OF AQUA TIC INSECTS. 737 



floated or sunk. If the wing of a dragon fly lie examined it appears to the 

 naked eye like fine gauze. If this be examined under a microscope with a ^ 

 objective a very wonderful and curious arrangement presents itself, i^ach of 

 the nervures are covered with curious protuberances exactly resembling thorns, 

 I have often wondered and puzzled as to what could be the use of these 

 " thorns " and I have now come to the conclusion that they act as a stiff support 

 to the more delicate portions of the wing and as an additional strength help- 

 ing the dragon fly to sustain its powerful flight. These '• thorns " ton may 

 cause the charactei-istic rustling of the wings when in flight. The win^s of 

 various gnats are fringed with long stiff hairs which may cause the piping 

 hum of these insects. The surface of the gnat's wing too is closely covered 

 with short dark hairs. A very similar arrangement is found on the wings of 

 Dytiscid beetles. These hairs may be of use as a protection against unr.eces- 

 sary damp, protecting the gnat from rain and damp atmosphere and the beetle 

 from getting the delicate membrane wet when submerged. These hairs can 

 only be seen under a high power of a microscope. 



In the legs of aquatic insects we find many wonderful and beautiful con- 

 trivances. In the larvae of some, these may either be well developed as in the 

 case of water beetles, May. Caddis, and dragon flies, merely taking the form of 

 hairs in gnats and entirely wanting in many aquatic Diptera. The 

 •• feathered " legs of various adult water beetles and water boatmen 

 {Corixa, Notonecta) are so arranged as to enable their owners to propel 

 their bodies with ease through the water. In the larger Dytiscid beetles the 

 fringe of hairs on the hind legs are remarkably long and stiff. The tarsi of 

 these limbs rotate upon their axis when the insect is swimming, and as Dr. 

 Sharpe remarks : — " In other words, what rowers call feathering the oar is 

 performed by the tarsus of the Dijtisckhe in a most perfect manner." Most 

 curious of all is the wonderful modification of the tarsus on the fore leg in 

 the males of many Dytiscid beetles in the form of suckers. It is now 

 generally supposed that these suckers are used for the firmer detention of the 

 female, and perhaps serve too in the capture of prey. 



One of the most beautiful of microscopic objects is a properly prepared fore- 

 leg of a Dytiscid beetle. On the undersurface of the circular disc are num- 

 bers of small stalked bodies, and on the first tarsal joint are seen two larger 

 structures. The sucker is composed of three joints and Ls fringed with stiff 

 hairs. On one species of Dytiscus no less than 170 sucking hairs have been 

 counted. Experiments have been made as regards the adhesive power of 

 these suckers, and it was found that a beetle by means of these could support 

 more than thirteen times its own weight. Another curious modification is 

 found in the hind limbs of whu-ligig {Gyrinidai) beetles. In these they 

 are short, broad and powerful, and their structure has been aptly likened to an 

 ivory memoranda tablet held together by a pin. so that they can either be 

 opened fan-wise or closed in a moment (Miall). The fore limbs in these 

 beetles are long and prehensile. Other aquatic insects have their fore limbs 



